Aspects of the present invention are directed to an underfill method and a chip package.
In the past, organic flip-chip packages have been fabricated with relatively small die sizes (i.e., <15 mm). These packages are still prevalent in industry, but there is a growing demand for packages with large die sizes (20 mm and larger). Additionally, the advancement of chip technologies from IBM's CMOS 7S to IBM's CMOS 12S technology involves the use of ultra-low K dielectrics, which are very fragile.
During thermal cycling, which is a component of most field use conditions in microelectronic applications, stresses develop due to the thermal expansion mismatch between the chip and the substrate. Historically, capillary underfill has been employed to mechanically couple the chip to the substrate, thus buffering the electrical interconnects (i.e., “controlled collapse chip connections”, C4s) between chip and substrate from these stresses. The magnitude of the stresses scale with dimension of the device. At some limit, the stresses may become sufficient to cause mechanical failure of underfill, and subsequent failure of the electrical interconnects of the device (C4 fatigue failure). The stress is at its largest magnitude at corners of the die. There, the stresses beyond a critical threshold may lead to interfacial adhesive failure between the underfill and the chip surface at the corner regions of the chip (i.e., “corner delamination”), or rupture of the underfill material itself (i.e., “corner cracking”), which in turn, may propagate in several directions; through the underfill, along the interface between underfill and the edge of the chip, along the interface between underfill and the face of the chip or into the active circuitry of the chip (which may include low-K layers).
Various methods have been proposed for their ability to alleviate corner cracking. Among the methods, one option is to match the coefficient of thermal expansion (CTE) of the laminate to the chip and would involve development and implementation of a new class of low expansion laminate materials. Another method is to reduce the ability of the crack delamination to be initiated. Methods to reduce crack initiation involve the use of dicing modifications. Yet another option is to reduce overall stress levels and may involve modifying the underfill, using different laminate/lid constructions, or limiting thermal cycling conditions. Reducing the ability of a crack to propagate has also been proposed. Methods to do this include modifying the underfills to improve adhesion.